User equipment (User Equipment, UE) that obtains no network service directly provided by an evolved universal mobile telecommunications system terrestrial radio access network (Evolved Universal Mobile Telecommunications System Terrestrial Radio Access Network, E-UTRAN) may use a device-to-device (device-to-device, D2D) technology to discover and connect to UE that provides a relay service. The UE that provides the relay service is also referred to as a proximity-based service user equipment-to-network relay (ProSe UE-to-NW relay). In this way, the UE that is not directly served by the E-UTRAN may obtain, in a relay manner, a network service provided by the E-UTRAN. The UE that accepts the relay service is also referred to as remote user equipment (remote UE).
Referring to FIG. 1, FIG. 1 is a schematic diagram in which a remote device receives, by using a relay device, a network service provided by an E-UTRAN. It can be learned from FIG. 1 that, the remote device establishes a connection to the relay device, the relay device is connected to a base station (for example, an eNodeB) by using a Uu interface, the base station is connected to an evolved packet core (Evolved Packet Core, EPC), and the EPC is connected to a public safety application server (Public Safety AS) by using an SGi interface. After the remote device establishes the connection to the relay device, a link between the remote device and the relay device may be referred to as a PC5 link. The remote device needs to measure signal strength of the PC5 link.
In a current D2D technology, two models in which remote UE accesses a ProSe UE-to-NW relay are defined, for example, are respectively referred to as Model (Model) A and Model B, and are separately described as follows:
Model A:
UEs in two roles are defined to execute a discovery process.
In terms of a first role, UE in the role may be referred to as announcing (Announcing) UE, and is configured to broadcast a specific message. UE interested in the message broadcast by the announcing UE may discover the announcing UE. In a relay model, a ProSe UE-to-NW relay may play the role of the announcing UE.
In terms of a second role, UE in the role may be referred to as monitoring (Monitoring) UE, and is configured to monitor a specific message broadcast by announcing UE in proximity. In the relay model, remote UE may play the role of the monitoring UE.
In Model A, all ProSe UE-to-Network relays may continuously broadcast announcement messages (Announcement message), and services that the ProSe UE-to-Network relays can provide may be indicated in the announcement messages. Each remote UE may select, for access according to the received announcement messages, a ProSe UE-to-Network relay that meets a condition (for example, can provide a service required by the remote UE).
In the prior art, after a ProSe UE-to-Network relay establishes a connection to remote UE, if the ProSe UE-to-Network relay cannot continue providing a relay service, the ProSe UE-to-Network relay may no longer broadcast an announcement message. Therefore, the remote UE cannot measure signal strength of a link between the remote UE and the connected ProSe UE-to-Network relay.
Model B:
UEs in two roles are also defined to execute a discovery process.
In terms of a first role, UE in the role may be referred to as discoverer (Discoverer) UE, and is configured to send a request message. The request message may include content in which the discoverer UE is interested. For example, the content may be a service required by the discoverer UE. In a relay model, remote UE may play the role of the discoverer UE.
In terms of a second role, the role may be referred to as discoveree (Discoveree) UE, is configured to monitor a request message sent by discoverer UE in proximity, and may reply with a specific response message if a request in a request message of one or some discoverer UEs is met. In the relay model, a ProSe UE-to-Network relay may play the role of the discoveree UE.
In Model B, remote UE first sends a solicitation request (Solicitation request) message, and a service required by the remote UE may be indicated in a solicitation request. Therefore, ProSe UE-to-Network relays that meet a condition requested by the remote UE (for example, can provide the service indicated in the solicitation request) may reply with response messages (Response message), and the remote UE selects a ProSe UE-to-Network relay for access from the ProSe UE-to-Network relays according to the received response messages.
In the prior art, if there is no connection request information of remote UE after a ProSe UE-to-Network relay establishes a connection to remote UE, the ProSe UE-to-Network relay no longer sends a response message.
Model A and Model B have the following problem:
Whether in Model A or Model B, it cannot be ensured that a ProSe UE-to-Network relay continuously sends a message (an announcement message in Model A or a response message in Model B) to remote UE after establishing a connection to the remote UE. Therefore, after the ProSe UE-to-Network relay establishes the connection to the remote UE, the remote UE cannot further measure signal strength of a PC5 link. Even if the signal strength of the current PC5 link is relatively weak, the remote UE neither can learn that the signal strength of the current PC5 link is relatively weak, and nor can reselect another ProSe UE-to-Network relay for access. Consequently, quality of a network accepted by the remote UE is relatively poor, and network use by the remote UE is affected. Even if there is a ProSe UE-to-Network relay that can provide relatively strong signal strength, the remote UE cannot perform access. In addition, link utilization is relatively low, overall network planning is improper, and system reliability is relatively low.